Specific, metabolism-driven transporters in excretory epithelia and barrier tissues play a important role in determining xenobiotic uptake, distribution and excretion. Along with xenobiotic metabolizing enzymes, these transporters are our first defense against chemical toxins. We use comparative models (renal proximal tubules from lower vertebrates, mammalian cells in culture derived from kidney and choroid plexus and intact mammalian and fish choroid plexus and brain microvessels) in combination with confocal microscopy, intracellular microinjection and isolated membrane vesicle techniques to define the cellular mechanisms that drive xenobiotic transport. Although work continues on the cellular and molecular biology of renal transport mechanisms, recent emphasis has been on development of imaging-based techniques to define mechanisms responsible for transport of foreign chemicals out of the central nervous system (CNS). First, we have used intact brain capillaries along with confocal imaging to obtain molecular and functional evidence in rat, pig and fish for involvement of the ATP-driven drug export pumps, p-glycoprotein and Mrp2, in the blood-brain-barrier. These multispecific transporters were immunolocalized to the luminal membrane of capilllary endothelial cells, where they were shown to transport a number of fluorescent xenobiotics, including the chemotherapeutic, daunomycin. In addition, use of capillaries isolated from fish brain have allowed us to greatly extend the time during which we can measure both the active transport and barrier functions of the vessels in response hormones and toxic agents. Second, we have begun to characterize xenobiotic transport mechanisms in intact rat choroid plexus using confocal microscopy. This tissue functions as the blood-cerebrospinal fluid (CSF) barrier that eliminates foreign chemicals from the CNS through organic anion and organic cation transport systems. Using fluorescent organic anions we have visualized all the steps in active transport from CSF to blood. We have confirmed the involvement of the transport protein, OAT1, in the uptake of organic anions from the CSF and have for the first time examined the mechanism by which organic anions are transported from choroid plexus cell to blood. The latter process is specific, concentrative and driven by the electrical potential difference across the basolateral membrane of the cells.